The present invention relates to a process for the electrochemical roughening of aluminum which can be used for printing plate supports, said process being performed by means of alternating current in an aqueous mixed electrolyte.
Printing plates (this term referring to offset-printing plates, within the scope of the present invention) usually comprise a support and at least one radiation-sensitive (photosensitive) reproduction layer arranged thereon, the layer being applied to the support either by the user (in the case of plates which are not pre-coated) or by the industrial manufacturer (in the case of pre-coated plates). As a layer support material, aluminum or alloys thereof have gained general acceptance in the field of printing plates. In principle, it is possible to use these supports without modifying pretreatment, but they are generally modified in or on their surfaces, for example, by a mechanical, chemical and/or electrochemical roughening process (sometimes also called graining or etching in the literature), a chemical or electrochemical oxidation process and/or a treatment with hydrophilizing agents. In modern continuously working high-speed equipment employed by the manufacturers of printing plate supports and/or pre-coated printing plates, a combination of the aforementioned modifying methods is frequently used, particularly a combination of electrochemical roughening and anodic oxidation, optionally followed by a hydrophilizing step. Roughening is, for example, carried out in aqueous acids, such as aqueous solutions of HCl or HNO.sub.3 or in aqueous salt solutions, such as aqueous solutions of NaCl or Al(NO.sub.3).sub.3, or also in combinations of these components, using alternating current. The peak-to-valley heights (specified, for example, as mean peak-to-valley heights R.sub.z) of the roughened surface, which can thus be obtained, are in the range from about 1 to 15 .mu.m, particularly in the range from 2 to 8 .mu.m. The peak-to-valley height is determined according to DIN 4768, in the October 1970 version. The peak-to-valley height R.sub.z is then the arithmetic mean calculated from the individual peak-to-valley height values of five mutually adjacent individual measurement lengths.
Roughening is, inter alia, carried out in order to improve the adhesion of the reproduction layer to the support and to improve the water/ink balance of the printing form which results from the printing plate upon irradiation (exposure) and development. By irradiating and developing (or decoating, in the case of electrophotographically working reproduction layers), the ink-receptive image areas and the water-retaining non-image areas (generally the bared support surface) in the subsequent printing operation, are produced on the printing plate and thus the actual printing form is obtained. The final topography of the aluminum surface to be roughened is influenced by various parameters, as is explained by way of example in the text which follows:
The paper "The Alternating Current Etching of Aluminum Lithographic Sheet", by A. J. Dowell, published in Transactions of the Institute of Metal Finishing, 1979, Vol. 57, pages 138 to 144, presents basic comments on the roughening of aluminum in aqueous solutions of hydrochloric acid, based on variations of the following process parameters and an investigation of the corresponding effects: The electrolyte composition is changed during repeated use of the electrolyte, for example, in view of the H.sup.+ (H.sub.3 O.sup.+) ion concentration (measurable by means of the pH) and in view of the Al.sup.3+ ion concentration, with influences on the surface topography being observed. Temperature variations between 16.degree. C. and 90.degree. C. do not show an influence causing changes until temperatures are about 50.degree. C. or higher, the influence becoming apparent, for example, as a significant decrease in layer formation on the surface. Variations in roughening time between 2 and 25 minutes lead to an increasing metal dissolution with increasing duration of action. Variations in current density between 2 and 8 A/dm.sup.2 result in higher roughness values with rising current density. If the acid concentration is varied in a range from 0.17 to 3.3% of HCl, only negligible changes in pit structure occur between 0.5 and 2% of HCl. Below 0.5% of HCl, the surface is only locally attacked and at the high values, an irregular dissolution of Al takes place. An addition of SO.sub.4.sup.2- ions or Cl.sup.- ions in the form of salts (e.g., by adding Al.sub.2 (SO.sub.4).sub.3 or NaCl) can also influence the topography of the roughened aluminum. Rectification of the alternating current shows that, obviously, both half-wave types are necessary to obtain a uniform roughening.
Thus, it can be assumed that the use of aqueous HCl solutions as electrolyte solutions for the electrochemical roughening of support materials made of aluminum is principally known. With these solutions it is possible--as is also evidenced by a great number of commercially available printing plates--to achieve a uniform graining, which is particularly suitable for applications in the field of lithography, the roughness values of which vary within a range which in general is appropriate for practical use. For certain applications of printing plates (for example, in the case of certain negative-working reproduction layers) there is, however, required a uniform surface topography showing relatively little depth of roughening, which is difficult to obtain in the known electrolyte solutions on a basis of aqueous HCl solutions using modern, high-speed apparatus. For example, the process parameters must be kept within very narrow limits, which involves a process that can be controlled with great difficulty only.
The influence of the electrolyte composition on the quality of roughening is, for example, also described in the following publications, in which aqueous mixed electrolytes are employed:
German Offenlegungsschrift No. 22 50 275 (equivalent to British Published Application No. 1,400,918) specifies aqueous solutions containing from 1.0 to 1.5% of HNO.sub.3 or from 0.4 to 0.6% of HCl and, optionally, from 0.4 to 0.6% of H.sub.3 PO.sub.4, for use as electrolyte solutions in the roughening of aluminum for printing plate supports, by means of alternating current;
U.S. Pat. No. 3,887,447 specifies aqueous solutions containing from 0.2 to 2% of HCl and from 0.15 to 1.5% of H.sub.3 PO.sub.4, for use as electrolyte solutions in the roughening of aluminum by means of alternating current;
U.S. Pat. No. 4,052,275 specifies aqueous solutions containing from 0.75 to 3.5% of HCl and from 0.2 to 1% of tartaric acid [2,3-dihydroxybutanedioic acid 1,4)] for use as electrolyte solutions in the roughening of aluminum;
U.S. Pat. No. 4,172,772 specifies aqueous solutions containing from 0.2 to 1.7% of HCl and from 0.5 to 4% of an alkanoic acid from C.sub.1 to C.sub.4 (particularly acetic acid, also known as ethanoic acid), for use as electrolyte solutions in the roughening of aluminum, by means of alternating current;
U.S. Pat. No. 4,367,124 specifies aqueous solutions containing from 0.35 to 3.5% of HCl and from 0.001 to 2% of a .beta.-dicarbonyl compound, such as acetylacetone or acetoacetic acid ethyl ester, for use as electrolyte solutions in the roughening of aluminum support materials for printing plates;
U.S. Pat. No. 4,339,315 specifies aqueous solutions containing from 0.1 to 1.0 mole/l of HCl and from 0.01 to 1 mole/l of citric acid or malic acid [3-hydroxy-pentanetrioic acid (1,3,5) and 2-hydroxybutanedioic acid (1,4)], for use as electrolyte solutions in the roughening of aluminum support materials for printing plates; and
U.S. Pat. No. 3,755,116 specifies as addition of anti-corrosive agents--including monoamines, diamines, aliphatic aldehydes, carboxylic acid amides, such as acetamide, urea, chromic acid and non-ionic surfactants, such as polyethylene glycol ethers or esters--to an aqueous HCl electrolyte, for roughening aluminum for printing plate supports.
The known organic additives to aqueous acid electrolytes, such as HCl or HNO.sub.3 solutions, have the disadvantage that in the case of high current loads (voltages) they become electrochemically unstable in modern continuously working web processing apparatus and decompose at least partially. The known inorganic additives, such as phosphoric acid, chromic or boric acid exhibit the disadvantage that quite often there is a local breakdown of their intended protective effect, as a consequence whereof single, particularly deep pits are formed at the respective spots.
In general, the known complex-forming additives accelerate the dissolution of the aluminum due to their "trapping" of released Al.sup.3+ ions and thus cause an increased roughening action. As a result thereof, quite often no creation of new pores is initiated, but pores which are already existent continue to grow, i.e., increased pitting occurs. It is true that usually the growth of individual pores is stopped relatively soon by the known inhibiting additives, and the formation of new pores can be initiated. These inhibitors exhibit, however, the decisive disadvantage that this protective effect can collapse due to voids, alloying constituents, and the like, so that single pores which are too deep are obtained on a surface which otherwise shows a shallow and uniform roughening. Support materials exhibiting these kinds of defects are not suitable for lithographic purposes.